The present invention relates to a coating composition which is suitable for coating surfaces, such as boards and in particular wood chipboard (particle board), which give off formaldehyde into the environment.
Wood chipboard is based on the bonding together of wood chips by glue. Glue is applied to the surface of the chips which are then bonded together while under pressure by the hardening of the glue. The glue in finished wood chipboard generally amounts to approx. 8-9% of the dry weight of the wood. The glues used are thermosetting resins the hardening of which is a chemical reaction and is not based on evaporation of a solvent. Hardening takes place rapidly.
The types of glue most commonly used by the wood chipboard industry are
carbamide glue PA1 phenolic glue PA1 melamine glue.
By far the most used in the manufacture of wood chipboard is carbamide glue, because its cost is comparatively low and its use is technically convenient.
Hardening of the glue sprayed onto the chips is achieved by means of a hardener, which is mixed with the glue shortly before use, and an elevated temperature.
A drawback of the boards made in this way has been found to be the release of formaldehyde, HCHO, from the glue, which is unpleasant and causes difficulties particularly for allergic people. There are several methods of binding the formaldehyde so released and thereby preventing its escape into the air of the room.
Most common is the use as a primer of an alkyld resin paint specially designed for priming wood chipboard, because this primer forms a fairly impervious film. The addition to paint of urea, H.sub.2 NCONH.sub.2, which reacts with the formaldehyde, is also known. Urea is admittedly cheap, but in order to react sufficiently a large quantity of it is required and this causes various surface defects in the paint film.
Hydrazines have also been used for this purpose (German Pat. No. 1 188 750), which are, however, very toxic and which do not remain in a reactive state in the paint for a sufficiently long time.